1. Field of the Invention
The present invention relates to the photolithography process used to manufacture semiconductor devices. More particularly, the present invention relates to a method of manufacturing a phase shift mask of a photolithographic exposure apparatus.
2. Description of the Related Art
A highly integrated semiconductor device is manufactured by selectively exposing a photoresist layer formed on a wafer to light projected through a reticle. The reticle bears a fine pattern that is to be transferred to the photoresist layer.
A phase shift mask is a reticle that includes both a light blocking layer and a light transmitting layer. Conventional phase shift masks include alternating phase shift masks and attenuated phase shift masks. The alternating phase shift mask comprises a 0° phase region and a 180° phase region, the latter of which is formed by etching a quartz substrate to a predetermined depth. The attenuated phase shift mask also comprises a 0° phase region and a 180° phase region, which are constituted by a phase shift layer on a substrate.
A method of manufacturing a conventional alternating phase shift mask will now be described referring to FIGS. 1A through 1D. First, as shown in FIG. 1A, a blocking layer is formed on a quartz substrate 10. A photoresist pattern (not shown) is formed on the blocking layer using a photolithography process. A blocking layer pattern 15 is formed by patterning the blocking layer so as to have a shape corresponding to that of the photoresist pattern. The blocking layer pattern 15 defines a 0° phase region ‘a’ and a 180° phase region ‘b’. The photoresist pattern is then removed. Next, another photoresist layer is formed on the quartz substrate 10, and the photoresist layer is exposed and developed so that a photoresist pattern 25 is formed. The photoresist pattern 25 exposes the 180° phase shift region ‘b’.
As shown in FIG. 1B, the quartz substrate 10 exposed by the photoresist pattern 25 is etched to a depth less than a predetermined depth ‘d’, wherein ‘d’ is the depth at which the mask would effect a phase shift of 180°. The reason why the quartz substrate 10 is first etched at phase region ‘b’ shallower than depth ‘d’ is as follows. Particles can be generated on the surface of the quartz substrate 10 during a photolithography process or an etching process for forming the photoresist pattern 25. If the quartz substrate 10 were subjected to the etching process while the particles were present on the surface of the substrate 10, the portion of the quartz substrate 10 under the particles would not be etched because the particles would act as a mask.
Isotropic etching has been suggested as a way to overcome this potential problem. However, isotropic etching results in the extension of an etching region and thus is difficult to adapt for use in forming a phase shift mask having a very fine pattern. Accordingly, when forming a conventional phase shift mask, a multi-step etching method is performed in which the quartz substrate 10 is anisotropically etched throughout part of its thickness, particles on its surface are removed by a cleaning process, and then the quartz substrate 10 is etched again. That is, the quartz substrate 10 is etched to a predetermined depth using the photoresist pattern 25, the photoresist pattern 25 is removed using a well-known method, and the surface of the resultant structure of the quartz substrate 10 is cleaned to remove the particles.
Then, as shown in FIG. 1C, another photoresist pattern 25 is formed on the quartz substrate 10 by a photolithography process to thereby expose the 180° phase shift region ‘b’. The quartz substrate 10, which has been already etched to a particular depth, is further etched at the 180° phase shift region ‘b’ using the second photoresist pattern 25 as a mask. The photolithography process, the etching process, and the cleaning process are repeated as many times as necessary until a trench 30 having the desired depth ‘d’ is formed at the 180° phase region ‘b’ of the quartz substrate 10, as shown in FIG. 1D.
Here, the predetermined depth ‘d’ of the trench 30, capable of shifting the phase of exposure light by 180°, is determined to satisfy the following mathematic formula 1:d=λ/2(n−1)
wherein λ denotes the wavelength of the exposure light and n denotes the refractive index of the quartz substrate 10. Accordingly, the predetermined depth ‘d’ of the trench 30 is dependent on the wavelength of light and the refractive index of the quartz substrate 10.
However, the multi-step etching method described above typically requires at least several rounds of the photolithography process, the etching process, and the cleaning process to form a 0° phase shift region ‘a’ and then a 180° phase shift region ‘b’ in a quartz substrate. Moreover, the photolithography process itself comprises a series of processes, i.e., coating, exposing, hardening and developing processes. At least one day is required to perform such a photolithography process just once. Accordingly, three to five days are required to manufacture a phase shift mask using the conventional multi-step etching method.